This invention applies to the separation of air by cryogenic distillation. Over the years, significant efforts have been devoted to improving the production process and lowering the cost of operation and equipment. One way to reduce costs of air separation units is to reduce the size and complexity of the equipment and piping systems.
Air is frequently separated by cryogenic distillation in a double column comprising the steps of feeding compressed, cooled, and purified air to a high pressure column where it is separated into a first nitrogen enriched stream at the top of the column and a first oxygen enriched stream at the bottom of the column. At least a portion of the first oxygen enriched stream is fed to a low pressure column to yield a second nitrogen enriched stream at the top and a second oxygen enriched stream at the bottom. A second oxygen enriched stream is separated at the bottom and a second nitrogen enriched stream is separated at the top of the low pressure column.
Air is sometimes separated by cryogenic distillation in a triple column comprising the steps of feeding compressed, cooled, and purified air to a high pressure column where it is separated into a first nitrogen enriched stream at the top of the column and a first oxygen enriched stream at the bottom of the column. At least a portion of the first oxygen enriched stream is fed to an intermediate pressure column to yield a second nitrogen enriched stream at the top and a second oxygen enriched stream at the bottom. At least a portion of the second nitrogen enriched stream is sent to a low pressure column or top condenser of an argon column, and at least a portion of the second oxygen enriched stream is sent to the low pressure column. A third oxygen enriched stream is separated at the bottom and a third nitrogen enriched stream is separated at the top of the low pressure column. Typically, the distillation columns are stacked on top of each other.
The nitrogen coming off the low pressure column (or the low pressure and intermediate pressure columns in the case of a triple column), which is very cold, is then removed from the separation system as product or waste gas. To assist in the separation and save on energy costs, the cold nitrogen streams are passed through a subcooler where distillation column liquids are cooled while heating the nitrogen before it is sent to the main heat exchanger. In the main heat exchanger, incoming air is cooled by the outgoing product and waste streams before being introduced into the cryogenic separation system. It is known to one of ordinary skill in the art that the main heat exchanger may be divided into two units wherein one unit contains the higher pressure gases and another contains the lower pressure gases.
U.S. Pat. Nos. 6,202,441, 6,276,170, 6,314,757 and 6,347,534, which are not admitted to be prior art with respect to the present invention, further describe the cryogenic separation processes known in the art and disclose information relevant to the cryogenic separation of air. However, these references suffer from one or more of the disadvantages discussed below.
The production capacities of modern air separation units continue to rise, thus units are becoming physically larger. Larger equipment and piping leads to layout, equipment, and piping design problems. For instance, a modern 5,000 ton per day unit may have a 72 inch line coming from the top of the low pressure column feeding the subcooler. As the nitrogen warms in the subcooler, it expands requiring an even larger line, 94 inch, exiting the subcooler. These large lines lead to very large cryogenic enclosures, and present significant thermal stress issues to designers. Furthermore, modern subcoolers are typically brazed fin exchangers of a highly compact design. Thus, the designer is faced with significant problems routing the large lines into and out of a single small, compact exchanger. Furthermore, the builder of the exchanger must mount larger headers on the brazed fin exchanger to facilitate receiving and discharging the nitrogen stream. These design issues lead problems with thermal stresses in the larger equipment pieces, higher equipment costs, and larger plant footprints.
Accordingly, it is a goal of the invention to provide a process design and apparatus configuration that allows the nitrogen leaving the cryogenic separation column to be separated into multiple streams feeding multiple subcoolers. By providing multiple subcoolers which cool different separation streams, the nitrogen flow is split and the line sizes are dramatically decreased. Correspondingly, the design problems and increased costs associated with the large piping and headers in the area of the subcooler are alleviated.
It is a further goal of the invention to simplify the piping and reduce equipment costs by integrating the subcoolers with corresponding main heat exchangers. By integrating the two, the piping between the subcooler and main heat exchangers may be eliminated.